A zeolite is a crystalline hydrated aluminosilicate that has a tetrahedra structure of the formula TO4. The tetrandra structure is in the form of a three-dimensional network that is formed by sharing oxygen atoms and wherein T represents tetravalent atoms, such as silicon, and trivalent atoms, such as aluminum.
In the crystalline structure of the zeolite there are pores and channels which may be interconnected. The dimensions and configuration of these pores and channels allow access of molecules of certain sizes or configurations, thus promoting the selectivity to certain products when the zeolite is used as a catalyst. Elements, such as metals, may also be deposited on or incorporated with the zeolite to facilitate selectivity or catalytic enhancement of the zeolite catalyst.
In certain instances phosphorus is used to modify the zeolite structure. In phosphorus treatment of zeolites, the zeolite is modified so that the pores and channels are occluded by various materials resulting from the treatment. This typically includes phosphates, phosphates bonded with non-framework aluminum and silicon, and amorphous alumina that is present as debris within the zeolite pore channels. Such catalysts have been shown to have improved catalysts characteristics in certain applications. Furthermore, the framework aluminum of the zeolite is decreased as a result of the phosphorus treatment and/or calcination.
The present invention is directed to a method of modifying phosphorus-containing or phosphorus-treated pentasil zeolite catalysts that have unique properties and characteristics that differ from conventional phosphorus-containing or phosphorus-treated zeolite catalysts.